Draw refractive index profile of a graded index fiber and show with neat diagram transmission of light through this fiber. Explain how GRIN fiber has transmission bit rate much higher than multimode step index fiber.

Mumbai University > Electronics and Telecommunication > Sem7 > Optical Communication and Networks

Marks: 5M

Year: May 2013

• The index of refraction varies smoothly and continuously over the diameter of core.

• Graded Index Fiber does not have a constant refractive index in the core. Due to this property they are also called inhomogeneous core fibers.

• It has a decreasing core index n(r) with radial distance from a maximum value of $n_1$ at the axis to a constant value $n_2$ beyond the core radius a in the cladding.

  The index variation is given by

Where, $\Delta$= relative refractive index difference

$\alpha$ = profile parameter which gives the characteristics refractive index profile of the fiber core.

= infinity, $ \ \ \ $ step index profile

$\alpha$= 2, $ \ \ \ $ parabolic profile

$\alpha$= 1, $ \ \ \ $ triangular profile

• The refractive index profile and ray transmission in a Multimode graded index fiber:

  • A multimode graded index fiber with a parabolic index profile core is illustrated in figure 12.

  • In multimode step index fiber core refractive index is constant therefore all modes have same velocity but travel different length this causes dispersion. Hence we change the velocity by changing the refractive index of core such that the time taken by all modes is same which gives zero dispersion.

  • It may be observed that the meridional rays shown appear to follow curved paths through the fiber core.

  • In graded index, core consists of different slabs of refractive indices from maximum value $n_1$ at the center to a minimum value at the core-cladding interface which causes the curved type light ray propagation.

• Graded index fiber has transmission bit rate higher than multimode step index fiber because the limitation in the bandwidth of a fiber due to dispersion is usually with regards to a RTZ (return to zero) code where the digital bit rate is equal to the bandwidth in hertz, Where as in NRTZ (Non return to zero) code maximum bandwidth is one half the maximum bit rate.

  • The pulse broadening is associated with fiber type.

  • Multimode step index fiber exhibits maximum dispersion of transmitted light pulse where as graded index fiber has better performance and hence has higher bit rate.